WASHINGTON — The White House said Thursday that U.S. intelligence agencies now assess, with “varying degrees of confidence,” that the Syrian government has used chemical weapons, but it said it needed conclusive proof before President Barack Obama would take action.

The disclosure, in letters to congressional leaders, takes the administration a step closer to acknowledging that President Bashar Assad has crossed a red line established by Obama last summer, when he said the United States would take unspecified action against Syria if there was evidence that chemical weapons had been used.

The White House emphasized that, “given the stakes involved,” the United States still needed “credible and corroborated facts” before deciding on a course of action. The letter, signed by the president’s director of legislative affairs, Miguel E. Rodriguez, said the United States was pressing for a “comprehensive United Nations investigation that can credibly evaluate the evidence and establish what happened.”

Although the White House said it could not confirm the circumstances in which victims were exposed to chemical weapons, it said it believed that the chemical agent sarin had been used. “We do believe,” the letter said, “that any use of chemical weapons in Syria would very likely have originated with the Assad regime.”

Britain, in a letter last month requesting a U.N. investigation, cited three suspected incidents in which chemical weapons were used: in a village west of the city of Aleppo and on the outskirts of Damascus, both on March 19, and in Homs on Dec. 24.

Secretary of State John Kerry, emerging from a congressional hearing, said the United States assessed that chemical weapons had been used in two instances, though he did not offer details.